In recent years, a substantial amount of research and engineering effort has been directed to the storage of electrical energy so that it would be available quickly and efficiently when needed, such as during high energy demand periods in the summer for air conditioning and in the winter for heating. It is also desirable to store electrical energy produced during the nighttime when consumption is low so that it is available for daytime use for peak shaving when demand is much greater, thereby permitting a power plant to run at a more uniform rate.
Electrical energy storage also may be used when it is desirable to generate power at a lower rate and perhaps at a different time than at which it will be consumed, store the generated power in the form of electrical energy and subsequently release the stored energy to meet high rate consumption demands.
One form of electrical energy storage which has been studied extensively is the superconducting magnetic energy storage (SMES) apparatus which is intended to operate at very low temperatures, i.e. cryogenic temperatures. One such system comprises a circular coil surrounded by a coil containment vessel containing liquefied helium at a temperature of 1.8.degree. K. The liquefied helium cools the coil to make it superconducting by lowering its electrical resistance. The coil containment vessel in turn may be surrounded by other structures such as a vacuum vessel and a shroud between the coil containment vessel and the vacuum vessel, but surrounding the coil containment vessel, to reduce heat transfer. The entire apparatus may be installed in a large circular trench or tunnel having inner and outer circumferential walls constructed to accept the radial loads applied during operation of the SMES apparatus.
Although very low temperature SMES apparatus and processes have been studied extensively using liquefied helium as the refrigerant or cooling liquid, there has been a continuing interest in SMES which utilizes high temperature superconducting (HTSC) materials which operate at higher temperatures, and particularly between the normal boiling point temperature of liquefied neon (27.09.degree. K.) and the normal boiling point temperature of liquefied nitrogen (77.36.degree. K.). Since a specific HTSC material may not likely give optimum SMES performance at the boiling point of liquefied neon or the boiling point of liquefied nitrogen, a cooling liquid is required which can supply the desired cooling for optimum performance within the temperature range from about 27.degree. K. to 77.degree. K. However, no single component cryogenic cooling liquid is available which is suitable for cooling a HTSC material to any specific optimum SMES performance temperature within the 27.degree. K. to 77.degree. K. temperature range.
The thermodynamic properties of single component cryogenic fluids which can be considered for use in cooling HTSC materials are summarized in Table 1.
TABLE 1 ______________________________________ Thermodynamic Properties Of Cryogenic Fluids Normal Cryo- Triple Boiling Critical Critical genic Point Point Point Point Fluid Temperature Temperature Temperature Pressure ______________________________________ Helium-4 -455.76.degree. F. -452.09.degree. F. -450.31.degree. F. 33.21 2.17.degree. K. 4.21.degree. K. 5.20.degree. K. psia n-Hydro- -434.56.degree. F. -422.97.degree. F. -399.95.degree. F. 190.75 gen 13.95.degree. K. 20.39.degree. K. 33.18.degree. K. psia Neon -415.49.degree. F. -410.90.degree. F. -379.66.degree. F. 394.73 25.54.degree. K. 27.09.degree. K. 44.45.degree. K. psia Nitrogen -346.01.degree. F. -320.41.degree. F. -232.50.degree. F. 493.00 63.15.degree. K. 77.36.degree. K. 126.21.degree. K. psia Carbon -337.02.degree. F. -312.74.degree. F. -220.43.degree. F. 507.44 Mon- 68.14.degree. K. 81.63.degree. K. 132.91.degree. K. psia oxide Argon -308.83.degree. F. -302.57.degree. F. -188.12.degree. F. 710.40 83.80.degree. K. 87.28.degree. K. 150.86.degree. K. psia Oxygen -361.84.degree. F. -297.35.degree. F. -181.08.degree. F. 736.86 54.35.degree. K. 90.18.degree. K. 154.77.degree. K. psia ______________________________________
The liquid temperature range of a specific cryogenic fluid listed in Table 1 extends from its triple point temperature to its critical point temperature. Thus, it is not possible to provide continuous liquid phase cooling of HTSC materials with the single component cryogenic fluids in Table 1 within the temperature range from about 27.degree. K. to 77.degree. K. and at about atmospheric pressure.
It is desirable that the cryogenic cooling liquid used to cool HTSC materials operate at a low non-vacuum pressure, i.e. near atmospheric pressure. Liquid cooling of HTSC materials in the lower portion of the temperature range from 27.degree. K. to 77.degree. K. could be effected by using high pressure liquid neon below its critical point temperature of 44.40.degree. K., but this would require a system that operates at high pressures, involving additional construction costs and operational problems. Likewise, liquid cooling of HTSC materials in the upper portion of the temperature range from 27.degree. K. to 77.degree. K. could be performed by using subcooled liquid nitrogen down to its triple point temperature of 63.15.degree. K., but this would require a system that operates under vacuum conditions, again involving additional construction costs and operational problems.
Although the above discussion has been directed specifically to SMES apparatus and methods, it applies equally well to the liquid cooling of apparatus utilizing HTSC materials for other purposes including superconducting power transmission lines, electric generators and transformers, levitation apparatus using superconducting magnets such as for a railroad train, electric motors, magnetic separators, fusion magnets, magnetic resonance apparatus, supercollider apparatus, electromagnetic resonators, superconductive transistors, superconductive microwave cavity filters, electronic systems and electromagnetic launching equipment such as for a railgun.
From the above it is clear that a need exists for a suitable mixed refrigerant or cooling liquid as well as for apparatus and methods which employ HTSC materials and the needed cooling liquid.